init: Don't look for logical partitions via uevents.

When using AVB, init's first stage regenerates uevent to find the paths
of block devices for verified partitions. If it can't find them all, it
will panic, causing the device to boot to recovery.

This does not work with logical partitions, since devices for those
partitions are created later in the first stage. In fact, they cannot be
created until uevent regeneration completes, since logical partitions are
are created by finding the "super" partition.

To address this we exclude logical partitions (as marked in fstab) from
the device finding process. Note that partitions moved from GPT to liblp
will no longer appear in by_name_symlink_map_.

Bug: 79173901
Test: AVB 2.0 device boots after deleting the vendor partition,
      creating a super partition, and flashing it with a dynamic
      vendor partition.

Change-Id: I19371b05912240480dc50864a2c04131258a7103